1. Field of the Invention
The present invention relates to an active matrix type display apparatus including luminescent elements such as EL (Electro-luminescence) elements or LED (Light Emitting Diode) elements which emit light by driving current flowing in thin films of organic semiconductors or the like, and also including thin film transistors (hereinafter TFT's) to control the emitting operation of these luminescent elements. More particularly, the present invention relates to a technique of driving each element formed in this type of display apparatus.
2. Description of the Related Art
Active matrix type display apparatuses incorporating luminescent elements of a current controlling type, such as EL elements and LED elements, have been proposed. Since any luminescent element employed in this type of display apparatus emits by itself, there are advantages in using no back-light and in having a minimal dependence on the viewing angle and the like.
FIG. 31 is a block diagram illustrating an active matrix type display apparatus incorporating organic thin film EL-elements of an electric charge filling type, as an example of these types of display apparatuses. In the display apparatus 1A shown in this figure, a plurality of scanning lines “gate”, a plurality of data lines “sig” extending in a direction that intersects the direction in which the scanning line “gate” extend, a plurality of common power supply lines “com” extending parallel to the data lines “sig”, and a plurality of pixels 7 located at the intersections of the data lines “sig” and the scanning lines “gate” which are formed on a transparent substrate.
Each pixel 7 comprises a first TFT 20 in which a scanning signal is supplied to the gate electrode (a first gate electrode) through the scanning gate, a holding capacitor “cap” which holds an image signal supplied from the data line “sig” via the first TFT 20, a second TFT 30 in which the image signal held by the holding capacitor “cap” is supplied to the gate electrode (a second gate electrode), and an luminescent element 40 (indicated as a resistor) into which the driving current flows from the common power supply line “com” through the second TFT 30.
In the above display apparatus 1A, both the first TFT 20 and the second TFT 30 are conventionally formed, as with an N channel type TFT or a P channel type TFT, as shown in an equivalent circuit diagram of FIG. 32, from the viewpoint of simplifying the production process, for example, in the case of an N channel type. Taking the N channel type as an example, as shown in FIGS. 33(A) and (B), when the high potential image signal “data” is written into the holding capacitor “cap” from the data line “sig”, while the scanning signal “Sgate” supplied through the scanning line “gate” has become higher in potential to turn the first TFT 20 “on”, the second TFT 30 is held in the “on” state. Consequently, in the luminescent element 40, the driving current keeps flowing from a pixel electrode 41 to an opposite electrode “op” in the direction indicated by the arrow “E” and consequently, the luminescent element 40 keeps emitting (the “on” state). On the other hand, when the image signal “data”, which is lower than the intermediate between the potential of the common power supply line “com” and the potential of the opposite electrode “op”, is written into the holding capacitor “cap” from the data line “sig”, while the scanning signal “Sgate” supplied through the scanning line “gate” has become higher in its potential to turn the first TFT 20 “on”, the second TFT 30 is turned “off” and consequently, the luminescent element 40 is turned “off” (the “off” state).
In the above display apparatus 1A, a semiconductor thin film, an insulating thin film, an electrode, etc., which constitute each element, are formed by thin films deposited on the substrate. Considering the heat resistance of the substrate, a low-temperature process is often used to form the thin films. Therefore the quality of the thin film is poor, as is evidenced by the frequent defects caused by a physical-property difference between a thin film and a bulk, which result in problems such as an electrical breakdown, and wherein time-degradation is apt to arise in the TFT and similar devices.
In the case of a liquid crystal display apparatus incorporating liquid crystals as light modulation elements, although it also uses the thin films, time-degradation can be suppressed not only in the liquid crystal but also in the TFT, because the light modulation element is driven by AC power. On the other hand, in the display apparatus 1A incorporating luminescent elements of the current controlling type, time-degradation is more often encountered in the TFT than in the liquid crystal display apparatus insofar as the apparatus is essentially driven by D.C. power. Although improvements have been made in the structure of the TFT and the process techniques in the display apparatus 1A, incorporating luminescent elements of the current controlling type, they do not yet seem to be improved enough.
In the case of incorporating the liquid crystals as the light modulation elements, the power consumption is small because the light modulation element is controlled by the voltage which causes the current flow in each element to be only momentary. On the other hand, in the display apparatus 1A incorporating luminescent elements of the current controlling type, a constant driving current is required to keep the luminescent element “on”, and this results in high power consumption and the risk of the frequent occurrence of electrical breakdown and time-degradation.
Further, in the liquid crystal display apparatus, the liquid crystal can be AC-driven by one TFT per one pixel. On the other hand, in the display apparatus 1A incorporating luminescent elements of the current controlling type, the luminescent element 40 is DC-driven by two TFTs 20, 30 per one pixel. This raises the driving voltage, and exacerbates the aforementioned problems, such as electrical breakdown and time-degradation. For example, as shown in FIG. 33(A), the gate voltage “Vgsw” of the first TFT, when selecting a pixel, corresponds to the potential difference between the potential equals to the higher potential of the scanning signal “Sgate” and the potential of the potential-holding electrode “st” (the potential of the holding capacitor “cap” or the potential of the gate electrode of the second TFT 30). Therefore when the potential of the potential-holding electrode “st” and, hence, the gate voltage “Vgcur” of the second TFT 30 are raised to make the luminescent element 40 emit in a high luminance, the gate voltage “Vgsw” of the first TFT 20 is lowered correspondingly. Therefore, the greater amplitude of the scanning signal “Sgate” has to be employed, requiring the higher driving voltage in the display apparatus 1A. Besides, in the aforementioned display apparatus 1A, since when the luminescent element 40 is “off”, the potential of the image signal “data” is made lower than the intermediate potential between the potential of the common power supply line “com” and the potential of the opposite electrode “op” in order to turn the second TFT 30 “off”, there is another problem of increased amplitude of the image signal “data”. Accordingly, in this display apparatus 1A, special consideration for the power consumption and the withstanding of voltage of the TFT, etc. is needed compared to the liquid crystal display apparatus. However, the conventional display apparatus 1A has not been provided with the sufficient consideration of these factors.